Emilie from DxO has commented on their site saying they "guess" it is a strong AA filter:

"Yes, this is something we checked many times before publish it,...Sharpness measured on GX7 is a bit low, but pretty close to the GX1 results. We guess there is a strong Anti Alias filter on this cameras."

It seems to me they did something wrong or had a bad unit.

Strong AA filter as an explanation for this kind of discrepancy (worst results for any MFT camera ever tested, including all those with 12 MP sensors)? No way. If that had actually been the case (which is of course extremely unlikely in the first place), it would have been known long ago. Several test sites (inluding SLR Gear/Imaging Resources and optyczne.pl, the Polish counterpart) have looked at the resolution of the sensor without finding anything unusual.

So DxO made a mistake or had a faulty unit. What is really weird is that they went ahead and published the results in spite of the fact that something is clearly wrong.

Checked.. DxO has tested 29 different mFT lenses on 13 different mFT cameras (+ a few older lenses only on older cameras), and with every single of the 29 lenses the GX7 gets a very low score. Makes me wonder... mistakes happen, but 29 times in a row! Could it be that they don't actually test every single lens on every single body, but instead use the test result from one camera to derive/calculate the 'results' for all the other cameras?

If doing it like that, then it's only necessary to test a single 'reference' lens on a new camera like the GX7. Saves a lot of work, but if something goes wrong, then all the derived results will be wrong too.